The (Al,Ga,In)N material system includes materials having the general formula AlxGayIn1-x-yN where 0≦x≦1 and 0≦y≦1. In this application, a member of the (Al,Ga,In)N material system that has non-zero mole fractions of aluminium, gallium and indium will be referred to as AlGaInN, a member that has a zero aluminium mole fraction but that has non-zero mole fractions of gallium and indium will be referred to as InGaN, a member that has a zero indium mole fraction but that has non-zero mole fractions of gallium and aluminium will be referred to as AlGaN, and so on. There is currently considerable interest in fabricating semiconductor light-emitting devices in the (Al,Ga,In)N material system since devices fabricated in this system can emit light in the blue-violet wavelength range of the spectrum (corresponding to wavelengths in the range of approximately 380-450 nm).
Semiconductor light-emitting devices fabricated in the (Al,Ga,In)N materials system are described, for example, by S. Nakamura et al in “Jap. J. Appl. Phys.” Vol. 35, pp L74-L76 (1996). They are also described in U.S. Pat. No. 5,777,350, which teaches use of the metal-organic chemical vapour deposition (MOCVD) growth technique to fabricate light-emitting devices in the (Al,Ga,In)N materials system. MOCVD (also known as metal-organic vapour phase epitaxy or MOVPE) takes place in an apparatus which is commonly at atmospheric pressure but sometimes at a slightly reduced pressure of typically about 10 kPa Ammonia and the species providing one or more Group III elements to be used in epitaxial growth are supplied substantially parallel to the surface of a substrate upon which epitaxial growth is to take place, thus forming a boundary layer adjacent to and flowing across the substrate surface. It is in this gaseous boundary layer that decomposition to form nitrogen and the other elements to be epitaxially deposited takes place so that the epitaxial growth is driven by gas phase equilibria.
Another known semiconductor growth technique is molecular beam epitaxy (MBE). In contrast to MOCVD, MBE is carried out in a high vacuum environment. In the case of MBE as applied to the (Al,In,Ga)N system, an ultra-high vacuum (UHV) environment, typically around 1×10−3 Pa, is used. A nitrogen precursor is supplied to the MBE chamber by means of a supply conduit and species providing aluminium, gallium and/or indium, and possibly also a suitable dopant species, are supplied from appropriate sources within heated effusion cells fitted with controllable shutters to control the amounts of the species supplied into the MBE chamber during the epitaxial growth period. The shutter-control outlets from the effusion cells and the nitrogen supply conduit face the surface of the substrate upon which epitaxial growth is to take place. The nitrogen precursor and the species supplied from the effusion cells travel across the MBE chamber and reach the substrate where epitaxial growth takes place in a manner which is driven by the deposition kinetics.